Carbohydrates

1
Carbohydrates

• Carbohydrates (or saccharides) consist of only
  carbon, hydrogen and oxygen
• Carbohydrates come primarily from plants, however
  animals can also biosynthesize them
• The Carbon Cycle describes the processes by
  which carbon is recycled on our planet
• - Energy from the sun is stored in plants, which
  use photosynthesis to convert carbon dioxide and
  water to glucose and oxygen
• - In the reverse process, energy is produced
  when animals oxidize glucose during respiration

2
Simplified Carbon Cycle
3
Types of Carbohydrates

• Monosaccharides are the simplest carbohydrates
• - Also called simple sugars
• - Cant be split into smaller carbohydrate units
• - Examples glucose, fructose, galactose,
  ribose
• Disaccharides are two monosaccharides bonded
  together
• - Can be split into two monosaccharides using an
  acid or enzyme catalyst
• - Examples sucrose (table sugar), lactose
  (milk sugar)
• Polysaccharides are polymers of monosaccharides
• - Used for storage of carbohydrates
• - Can be split into many monosaccharides with
  acid or enzymes
• - Examples starch, cellulose, glycogen

4

• monosaccharides have the general formula CnH2nOn,
  where n varies from 3 to 8

5
Classification of Monosaccharides

• Monosaccharides have 3-8 carbons in a chain, with
  one carbon in a carbonyl group, and the other
  carbons attached to hydroxyl groups
• - An aldose has the carbonyl C1 (an aldehyde)
• - A ketose has the carbonyl on C2 (a ketone)
• - The number of carbons is indicated as follows
  triose (3 Cs), tetrose (4 Cs), pentose (5
  Cs), hexose (6 Cs)

6
Monosaccharides

• Monosaccharides are classified by their number of
  carbon atoms

7
Monosaccharides

• Fischer projection a two dimensional
  representation for showing the configuration of
  tetrahedral stereocenters
• horizontal lines represent bonds projecting
  forward
• vertical lines represent bonds projecting to the
  rear

8
D and L Sugars and Fischer Projections

• Monosaccharides are chiral compounds (have
  stereoisomers)
• - Each monosaccharide has two enantiomeric forms
• The D and L classifications are based on
  glyceraldehyde
• - Each enantiomer refracts plane-polarized light
  in equal magnitude but opposite direction
• - In glyceraldehyde, L rotates light to the left
  and D to the right (however, this is not true for
  all sugars)
• L-glyceraldehyde has the hydroxyl group on the
  left, and D-glyceraldehyde has the hydroxyl group
  on the right
• - All other sugars are classified based on the
  position of the hydroxyl group farthest from the
  carbonyl

9
(No Transcript)
10
(No Transcript)
11
(No Transcript)
12
D,L Monosaccharides

• the most common D-tetroses and D-pentoses

13
Three Important Monosaccharides

• D-Glucose is the most common monosaccharide
• - Primary fuel for our cells, required for many
  tissues
• - Main sources are fruits, vegetables, corn
  syrup and honey
• - Blood glucose is maintained within a fairly
  small range
• - Some glucose is stored as glycogen, excess is
  stored as fat
• D-Galactose comes from hydrolysis of the
  disaccharide lactose
• - Used in cell membranes of central nervous
  system
• - Converted by an enzyme into glucose for
  respiration (lack of this enzyme causes
  galactosemia, which can cause retardation in
  infants if not treated by complete removal from
  diet)
• D-Fructose is the sweetest carbohydrate
• - Converted by an enzyme into glucose for
  respiration
• - Main sources are fruits and honey
• - Also obtained from hydrolysis of the
  disaccharide sucrose

14
Structures of Glucose, D-Galactose and D-Fructose

• Note that in nature, only the D enantiomers of
  sugars are used
• What is the relationship between D-glucose and
  L-glucose?
• What is the relationship between D-glucose and
  D-galactose?
• What is the relationship between D-glucose and
  D-fructose?
• (enantiomers, diastereomers and constitutional
  isomers)

15
Amino Sugars

• Amino sugars contain an -NH2 group in place of an
  -OH group
• only three amino sugars are common in nature
  D-glucosamine, D-mannosamine, and D-galactosamine

16
Cyclic Structures of Monosaccharides

• Recall that an alcohol can react with an aldehyde
  or ketone to form a hemiacetal
• If the alcohol and aldehyde or ketone are in the
  same molecule, a cyclic hemiacetal is formed
• Monosaccharides in solution are in equilibrium
  between the open-chain and ring forms, and exist
  primarily in the ring form
• (Why does the 6-membered ring form, instead of a
  smaller one?)

17
Cyclic Structure

• Aldehydes and ketones react with alcohols to form
  hemiacetals

18
Drawing Haworth Structures for Cyclic Forms

• Step 1 Number the carbons in the chain and turn
  the Fischer projection of the open-chain form
  clockwise 90 degrees
• - Hydroxyl groups that were on the right are now
  on the bottom, and hydroxyl groups that were on
  the left are now on the top (they will stay on
  bottom or top in the Haworth structure)
• Step 2 Rotate around so that C-6 sticks up from
  C-5, and the hydroxyl group on C-5 points towards
  C-1
• Step 3 Form the cyclic hemiacetal by bonding
  the hydroxyl O to the carbonyl C and moving the
  hydroxyl H to the carbonyl O
• Note the cyclic hemiacetal has a new chiral
  carbon at C-1
• - The two possible stereoisomers are called
  anomers
• - The alpha anomer has the hydroxyl group down
• - The beta anomer has the hydroxyl group up

19
Haworth Projections

• In the terminology of carbohydrate chemistry,
• b means that the -OH on the anomeric carbon is on
  the same side of the ring as the terminal -CH2OH
• a means that the -OH on the anomeric carbon is on
  the side of the ring opposite from the terminal
  -CH2OH
• a six-membered hemiacetal ring is called a
  pyranose, and a five-membered hemiacetal ring is
  called a furanose

20
Haworth Projections

• D-Glucose forms these cyclic hemiacetals

21
Haworth Projections

• aldopentoses also form cyclic hemiacetals
• the most prevalent forms of D-ribose and other
  pentoses in the biological world are furanoses

22
Mutorotation

• When in solution, monosaccharides exist in
  equilibrium between the alpha and beta forms
• Each form is in equilibrium with the open-chain
  form (the process by which it goes back and forth
  is called mutorotation)
• Each time the chain closes, it can go to either
  form, although it turns out that beta forms more
  often (64 for glucose)

23
Chair Conformations

• For pyranoses, the six-membered ring is more
  accurately represented as a chair conformation

24
Chair Conformations

• in both a Haworth projection and a chair
  conformation, the orientations of groups on
  carbons 1- 5 of b-D-glucopyranose are up, down,
  up, down, and up